Tank for a swimming pool or the like

ABSTRACT

A tank of the swimming-pool type formed of rigid plastic material such as polyurethane foam and provided with a fluidtight lining of small thickness is placed directly in contact with the soil within a pit which has been dug in the ground. The modulus of elasticity of the side wall and preferably of the bottom is close in value to that of the surrounding soil so as to result in a uniform stress distribution and a deformability which removes any danger of crack formation.

ilite tates atet Friedland et al.

145] Oct. 15, 1974 TANK FOR A SWIMMING POOL OR THE LUKE lnventors:Jacques Friedland, 14, rue

Beautreillis; Pierre Habit), 2, rue Turgot, both of Paris, Seine, FranceFiled: May 21, 1973 Appl. No.: 362,236

Foreign Application Priority Data May 30, 1972 France 72.19256 US. Cl.52/1169, 4/172, E04h/3/l6 Field 01 Search 52/169, 309, 265, 167; /172References Cited UNITED STATES PATENTS Klingberg 52/169 X 3,015,1911/1962 Lucchesi 52/169 3,031,801 5/1962 Leuthesser 52/169 X 3,429,0352/1969 Stillman t 52/169 3,610,564 10/1971 Mattingly...; 52/169 X3,660,957 5/1972 Schankler 52/169 X Primary Examiner-1r. Faw Attorney,Agent, or FirmThomps.on and Thompson [57] ABSTRACT A tank of theswimming-pool type formed of rigid plastic material such as polyurethanefoam and provided with a fluid-tight lining of small thickness is placeddirectly in contact with the soil within a pit which has been dug in theground. The modulus of elasticity of the side wall and preferably of thebottom is close in value to that of the surrounding soil so as to resultin a uniform stress distribution and a deformability which removes anydanger of crack formation.

2 Claims, 5 Drawing Figures 1 TANK FOR A SWIMMING POOL OR THE LIKE Thisinvention relates to a swimming-pool tank which is placed in a pit dugin the ground. The word tank as used in the description of the presentinvention refers preferentially to-swimming-pool's but extends to alltypes of receptacles for containing liquids, especially for industrialor agricultural purposes.

It is known that a swimming-pool tank has a side wall and a bottom whichare adjacent to the soil of the pit and the function of which is tomaintain the shape of the tank while affording resistance to the thrustdeveloped by the hydrostatic pressure of the liquid.

Said side wall and tank bottom are usually covered with a fluid-tightlining.

In one known design of swimming-pool tank, the bottom is a concrete raftand the side wall is constructed by means of breeze-blocks or concreteslabs which are bonded together by means of mortar and covered with afluid-tight lining which is moulded over the subjacent slabs orbreeze-blocks.

Swimming-pool tanks of known types are capable of undergoing smalldeformations at right angles to the surface of the side wall or thebottom without thereby sustaining any damage, under the action ofvariations which occur in the pressure exerted by the surrounding media,especially at the time of emptying and filling of the tank orduringperiods of frost.

The masonry or concrete slabs of which the side wall and the bottom ofthe tank are formed have the disadvantage of being non-deformable alongtheir own plane, with the result that any possible deformation under theinfluence of the above-mentioned variations in pressure is liable totake place only at the level of localized zones of lower resistance suchas the joints between the slabs or breeze-blocks. In consequence, thefluid-tight lining which is bonded to the slabs is subjected at thelevel of said joints to tensile stresses which are liable to result incrack formation or tearing. Said fluid-tight lining must therefore beformed of material which is capable of withstanding such stresseswithout damage.

In the most recent designs, this problem is solved by making use of acomposite material which has both.

flexibility and strength but is very costly, such as a laminate of glassfabrics impregnated with polyester resin.

Elements of large size are sometimes employed with a view to limitingthe number of joints between the breeze-blocks or slabs since thesejoints are liable to constitute an equal number of zones of lowerstrength. However, both labour and handling means are required for thepositioning of these elements.

Furthermore, the walls of the above-mentioned swimming-pool tanksprovide only limited thermal insulation for the water which is containedin the tank and is usually heated to a temperature above that of thesurrounding soil.

This invention is intended to overcome the disadvantages referred-toabove by providing a swimming-pool tank of simple and inexpensiveconstruction comprising a wall adjacent to. the soil andhaving adeformability such as to eliminate any risk of crack formation ordamaging of the tank as a result of the variations in pressure whichwere mentioned earlier.

In accordance with the invention, the swimming-pool tank placed in a pitand comprising a wall which is adjacent to the soil of the pit andcovered with a fluid tight lining is characterized in that the wall hasa modulus of elasticity which is closely related in value to the modulusof elasticity of the soil in the vicinity of the pit.

It is known that the modulus of elasticity E which is also designated asthe coefficient of elasticity or Youngs modulus of a material isdetermined by crushing a cylindrical test specimen of said materialbetween two plates and by measuring the deformation exhibited by thetest specimen as a function of the pressure applied.

If I is the initial length of the test specimen and d! is thedeformation developed at the pressure P, the modulus of elasticity E isequal to: P-l/dl. V

.The modulus of elasticity of a material is therefore equal to thequotient of the pressure applied on the material by the correspondingrelative deformation and consequently has the dimensions of a pressure.

p The modulus of elasticity is a physical quantity which characterizes amaterial in a very accurate manner. In fact, the modulus of elasticityvaries considerably from one material to another as shown by thefollowing list of materials for whichthe moduli of 'elasti city'aregiven in decanewtons per cm (daN/cm which is a unit corresponding to 1bar):

. gelatin- 0.1 daN/cm pneumatic type rubber 5O methyl polymethacrylate30,000 do; tungsten carbide 3,000,000 do.

The modulus of elasticity is also variablefrom one soil to another. Afew moduli of elasticity of different soils are given hereunder:

20 to I00 Surface clays daN/cm Silts 40 to I50 do.

Sands to 400 do.

Old alluvia, sands and gravels 400 to 2000 u do. do.

Chalk 10,000 to 20,000 i As contemplated by the invention, theadaptation of the modulus of elasticity of the wall to that of the soilsurrounding the pit results in the following technical effect: when thewall of the tank in accordance with the invention is subjected tovariations of pressure, which takes place in practice when the tank isemptied and filled, the soil which is adjacent to said wall will be deformed in much the same manner as this latter. In consequence, thetensile stresses which are liable to be set up in the plane of said wallunder the action of said variations of pressure can only be verymoderate and are not liable to cause crack formation in the wall.

In a preferred embodiment of the invention, the entire wall includingthe bottom of the tank has a modulus of elasticity which is close invalue to that of the soil. In this manner, even the most moderatetensile stresses are distributed throughout the entire tank.

In order to remove any danger of crack formation in the tank wall, themodulus of elasticity of said wall is preferably equal at a maximum totwice themodulus of elasticity of the soil.

For example, when the tank wall is adjacent to a soil having a modulusof elasticity within the range of 50 to daN/cm, which corresponds to agood soil for the installation of a swimming-pooltank, it is preferableto ensure that the modulus of elasticity of said wall is within therange of 50 to 200 daN/cm The aforementioned conditions in regard to themodulus of elasticity of the tank wall make it possible for this latterto undergo deformations in its own plane without any resultant localizedconcentration of stresses which would be liable to produce cracks.

However, the wall must also be capable of undergoing deformations atright angles to its own plane without cracking.

In point of fact, a soil is never homogeneous and its modulus ofelasticity is variable from one point to another.

It can be acknowledged that, by reason of this lack of homogeneity ofthe soil and under the action of pressures applied at right angles tothe wall, this latter must be capable of undergoing a deflection (f) of1 cm over a distance (2a) of l m.

Given that R is the radius of curvature corresponding to said deflectionf and said distance 20, we have:

f R w) that is to say for a small value off:

R=a 2f= 1,250 cm In the case of a wall having a thickness e, failure isobtained when its radius of curvature attains the value defined by theknown relation:

e 2,500 s/E In practice, in order to guard against any risk of crackformation, it is preferable to limit the thickness of the wall to avalue which is four times smaller than that given by the formulareferred-to above, that is to say:

e A 2,500 s/E This relation defines the maximum limit which is set forthe thickness of the wall and which must not be exceeded.Fluid-tightness of the swimming-pool tank in accordance with theinvention can be obtained by covering the wall with an inexpensivelining which can be of very small thickness since it will practicallynot be subjected to any tensile stress.

Further properties of the invention will become apparent from thedetailed description which is given below, reference being made to theaccompanying drawings which are given by way of non-limitative example,and wherein:

FIG. 1 is a perspective view of a swimming-pool tank in accordance withthe invention, the side walls of which are rounded;

FIG. 2 is a sectional view of the swimming'pool tank as taken along lineII-ll of FIG. 1;

, FIG. 3 is a sectional view of the tank wall which is drawn to a largerscale and shows a particular method of anchoring said wall to theground;

FIG. 4 is a sectional view of the upper edge of the swimming-pool tank;

FIG. 5 is a sectional view of the tank wall which is drawn to a largerscale and shows in particular the structure of a fluid-tight lining.

Referring now to FIGS. 1 and 2, it is apparent that the swimming-pooltank 1 in accordance with the invention is placed within a pit dug inthe ground 2. The tank 1 has a side wall 3 of rounded shape and a bottom4 which are adjacent to the soil and covered with a fluid-tight lining5.

In accordance with the invention, the wall 3 and the bottom 4 of thetank I are formed by means of material having a modulus of elasticitywhich is close in value to that of the ground or soil 2 and preferablyequal at a maximum to twice that of the soil. To this end, the value ofthe modulus of elasticity of the soil is determined by the methodindicated earlier, namely by means of test specimens taken from theimmediate vicinity of the pit and the mean value of the modulus E isthen determined.

Under these conditions, the tensile stresses which are generated withinthe wall 3 and the bottom 4 under the effect of variations of pressurewhich occur, for example, at the time of filling or emptying of the tank1 remain of very moderate value.

These stresses would be practically eliminated if it were found possibleto construct a wall 3 and a bottom 4 having a modulus of elasticitywhich is equal to that of the soil 2 but this condition cannot befulfilled systematically since the modulus of elasticity of the soil canvary to an appreciable extent from one place to another.

Taking account of the fact that swimming-pool tanks are usually placedin a soil having a modulus of elasticity within the range of 50 todaN/cm it is an advantage to form the wall 3 and the bottom 4 of thetank 1 in a material having a modulus of elasticity which issubstantially comprised between 50 and 200 daN/cm As shown in FIG. 2,the side wall 3 and the bottom wall 4 of the tank 1 are formed by acontinuous layer of material whichcomplies with the above-mentionedcharacteristics of elasticity, with the result that the entire wall ofthe tank 1 has a modulus of elasticity which corresponds substantiallyto that of the soil.

Accordingly, the low stresses which are liable to be set up as a resultof a variation of the pressure applied on the layer aforesaid areuniformly distributed throughout all the tank walls without giving riseto any appreciable stress concentration at a given point of said walls.

Suitable materials corresponding to the characteristics of elasticityaforesaid are selected from the plastics of cellular structure such asthe polyurethane foams, expanded polystyrene and polyethylene whichadditionally ensure good thermal insulation for the water contained inthe tank. The foams which usually permit achievement of the best resultsare the rigid polyurethane foams which have a specific density withinthe range of 0.03 to 0.15 g/cm and have a modulus of elasticity afterhardening which ranges from 50 to 200 daN/cm.

By way of example, it is possible to make use of a polyurethane foam ofthe type commercialized by Societe Bayer under the trade name ofMoltopren. It is also possible to employ coating or sealing compounds ofthe mastic type containing a polyurethane resin binder and filled withexpanded vermiculite, sawdust or fine rubber powder so as to form amaterial having the requisite characteristics. By suitably selecting theproportions of constituents, it is an easy matter to modify the modulusof elasticity of the product so as to attain the desired value.

In the swimming-pool tank according to the invention, it is unnecessaryand even disadvantageous to provide a substantial thickness in the caseof either the wall 3 or the bottom 4. A thickness of a few millimetresor a few centimetres at a maximum is sufficient. This particular featurewhich is a priori contrary to ordinary experience represents one of theessential advantages of the invention.

The thickness of the polyurethane layer 3 has been exaggerated in thedrawings, especially in FIG. 4, for the sake of enhanced clarity ofillustration. It is readily apparent that the real thickness of the postor beam 6 is distinctly greater than the thickness of the foam layer Inpoint of fact and in accordance with the invention, the maximumthickness of the wall 3 and the bottom 4 as expressed in centimetres ispreferably smaller than or equal to: A 2,500 -s/E.

From this it follows that, in the case of material having a modulus ofelasticity E equal to 200 daN/cm and a tensile strength s equal to 2daN/cm the maximum thickness of the wall 3 and the bottom 4 is equal toapproximately 6 cm. It is not essential although preferable to ensurethat the surface of the soil 2 which is adjacent to the wall 3 or thebottom 4 is perfectly levelled or made smooth since the application of alayer of plastic material such as the above-mentioned polyurethane foamseither by spraying or injection behind shuttering serves to compensatefor any surface irregularities of the soil as shown in FIG. 3.

However, in order to ensure good seating on the ground or soil 2, thesurface of this latter should preferably be as flat as possible. Thesurface flatness of the bottom of the pit can be obtained simply bymeans of a surface coating 12 of mortar as indicated in FIG. 2. Thesurface flatness of the sides of the pit can also be ensured by fillingthe largest cavities such as the cavi ties 13 (shown in FIG. 2) with acoating compound or with mortar.

As shown in FIG. 2 and more clearly in FIG. 4, the upper end of the wall3 of the tank 1 is applied against a rigid support such as a post 6 orany suitable material which is fixed in the ground at the edge 7 of thetank. A curbstone 8 which is usually of reinforced concrete covers thepost 6 as well as the upper end of the wall 3. The fluid-tight lining 5partially covers the curbstone 8. A concrete pavement 9 which is placedover said curbstone and over the upper end of the lining 5 plays acontributory part in anchoring this latter.

Adhesion of the end of the wall 3 to the post 6 and to the curbstone 8is obtained either by bonding or directly when the wall 3 is formed byapplication of a foam such as a ployurethane foam. Attachment of thecurbstone 8 to the post 6 and of the pavement 9 to the curbstone 8 isobtained by means of mortar.

Additional posts (not shown) which are similar to the post 6 can beplaced at intervals along the edge 7 and around the pit so as to ensurethat the wall 3 is securely amchored to the edge 7 of the swimming-pooltank. A construction of this type is advantageous in the case of aswimming-pool which has a sinuous contour. In the case of aswimming-pool of rectangular or polygonal shape, the posts 6 can bereplaced by a string of joists or beams, for example of reinforcedconcrete.

From FIG. 3 it is apparent that, in order to improve the adhesion of thewall 3 to the soil 2, anchoring rods or pins 10 of metal which isprotected against corrosion or of plastic material can be driven intothe ground to an appreciable depth, the ends ll of said rods beingembedded in the material which forms the wall 3. The pins 10 thus ensureeffective anchoring of the wall 3 relatively to the ground.

The continuous layer which forms the wall 3 and the bottom 4 of the tankis covered with a fluid-tight lining 5 made up of one or a number oflayers. Said fluid-tight lining does not need to be formed ofhigh-strength material such as glass fabric impregnated with polyesterresin. By way of example, a simple coating of polyurethane resin havinga thickness of a few millimetres and formed by means of a mixture ofproducts such as those made available by Societe Bayer under the tradenames Desmophen" and Desmodur may prove sufficient for this purpose.

Said fluid-tight lining can also be composed of three layers ofpolyurethane resin having different functions as shown in FIG. 5. Inthis figure, the first layer 5a which is illustrated is formed by anon-pigmented polyurethane resin which ensures fluid-tightness. Thelayer 5a is applied against the external surface of the wall 3 and ofthe bottom 4. The second layer 5b can consist of a polyurethane resin inwhich there have been incorporated coloured pigments, non slip pigmentsor simple fillers of silica. The external finishing layer 50 can be ofnon-pigmented polyurethane resin.

Prior to application of the fluid-tight lining, the surface of thepolyurethane foam layer can be trued by sand-papering.

Two examples of composition for a fluid-tight lining layer are givenhereunder:

Example I Sealing compound Polyhydroxylated polyester resin Desmophen850" Castor oil Titanium oxide Pyrogenated silica parts by weight do.

do. do.

Example ll Finishing coat over sealing compound Polyhydroxylatedpolyester resin Desmophen 850" I5 Castor oil 85 Titanium oxide 75Pyrogenated silica 15 parts by weight do.

do. do.

Prior to use, there are added to this composition 48 parts by weight ofDesmodur L 75", parts by weight of semi-rigid isophthalic unsaturatedpolyester resin and 0.6 parts by weight of monomer styrene.

Example lll Sealing compound 2 Semi-rigid isophthalic Prior to use,there is added to this composition a hardener formed by a diisocyanatewhich is dissolved in ethyl acetate such as Desmodur L 75 in aproportion of parts by weight of the composition aforesaid.

Example lV Finishing coat on sealing compound Semirigid isophthalicunsaturated polyester resin 48 parts by weight Polyhydroxylatedpolyester resin 13 do. Titanium oxide l2 do. Silica passed through alZO-micron mesh sieve do. Ultra-fine pyrogenated silica 2 do. Styrene 5do.

Prior to use, there is added to this composition a hardener formed by adiisocyanate which is dissolved in ethyl acetate such as Desmodur N 75in a proportion of 10 parts by weight of the composition aforesaid.

the walls are formed of a material which affords excellent thermalinsulation and said material can readily be placed in position on thesoil surface of the pit which is dug in the ground, thus permitting areduction in costs arising from the use of handling means and in labourcosts.

The invention is clearly not limited to the embodiments' which have beendescribed in the foregoing and a large number of alternative forms ofconstruction can accordingly be contemplated without thereby departingeither from the scope or the spirit of the invention.

In particular, the shape of the tank, the material or materialsconstituting said tank, the thickness of the walls, the anchoring ofsaid walls to the ground of the pit in which the tank is placed can beadapted to various uses.

In a particular mode of execution of the invention, it can also beensured that the constituent material of the tank wall is selected sothat, in addition to the requisite values of the modulus of elasticity,said material has a coefficient of expansion which is comparable withthat of the soil. This makes it possible to reduce the thermal stresseswhich arise either from the water or from ambient conditions.

We claim:

1. A swimming-pool tank in a pit in the ground with a wall adjacent tothe soil of said pit and having a fluidtight inner lining, wherein thevalue of the modulus of elasticity of said wall is equal at amaximumvalue to twice the modulus of elasticity of the soil in the vicinity ofsaid pit, wherein the maximum thickness 2 of the wall in centimeters isabout e A- 2,500- s/E of 50 to 200 daN/cm

1. A swimming-pool tank in a pit in the ground with a wall adjacent tothe soil of said pit and having a fluid-tight inner lining, wherein thevalue of the modulus of elasticity of said wall is equal at a maximumvalue to twice the modulus of elasticity of the soil in the vicinity ofsaid pit, wherein the maximum thickness e of the Wall in centimeters isabout e 1/4 . 2,500. s/E where E is the modulus of elasticity and s theultimate tensile strength of said wall, said wall further comprisingrigid anchoring means embedded therein and projecting into the soil ofsaid pit.
 2. A swimming-pool tank according to claim 1 and in which thewall is adjacent to soil having a modulus of elasticity within the rangeof 50 to 100 daN/cm2, wherein the wall is made of rigid polyurethanefoam having a specific density within the range of 0.03 to 0.15 g/cm3and a modulus of elasticity within the range of 50 to 200 daN/cm2.